This invention relates to cable television networks and more particularly to high-speed data communications over cable television networks. Even more particularly, the invention relates to real time device polling for high-speed data communications over cable television networks.
Much research and development has been done in recent years in the cable television industry towards utilizing existing television cable networks for other purposes. Such purposes include telephone communications, accessing the Internet with personal computers, and other types of high-speed data communication. This interest is due in part to the broad bandwidth of cable television networks, the associated lower costs, and increased competition in the communications industry. Television cable networks may be all coaxial cable or hybrid-fiber/coax (HFC). This new bi-directional traffic over the television cable network can be statistically multiplexed with the existing traffic.
There are other advantages as well. Utilizing routing technologies and real time service over an Internet Protocol (IP) network, a telephone connected to a cable modem at the customer location can send voice data upstream over the television cable network to the cable network head end. At the cable network head end, the voice data received from the customer location can be connected to an IP backbone. Voice data is returned over the IP backbone to the cable network head end and back over the television cable network to the cable modem and telephone at the customer location. In a similar fashion a personal computer at the customer location, having a cable modem connecting it to the television cable network, can send data to the cable network head end, access the Internet, and receive data back from the cable network head end over the television cable through the cable modem to the personal computer.
Digital voice and video data can be characterized as a periodic constant bit rate (CBR) data source that is either on or off. To get the largest benefit from statistical multiplexing, when periodic CBR data, such as voice data from a telephone conversation, is in an off period, bandwidth needs to be allocated to some other service. However, voice data on periods are of variable length, and difficult to predict. Traditional approaches have required heavy overhead during off periods, where checking for bursts of voice data occurs, in order to reduce delay when an on period begins. This heavy overhead approach reduces statistical multiplexing gain. Delay must be minimized wherever it occurs because of the accumulated effects that delay has all along the communication path from end to end. Delay values in the 200 msec to 500 msec range, which are common in satellite telephone calls, are very noticeable.
It is thus apparent that there is a need in the art for an improved method or apparatus for transmitting periodic CBR data, such as voice data, over a television cable network which will maximize statistical multiplexing gain by minimizing overhead during voice data off periods, and yet respond quickly when a voice data on period begins in order to minimize delay. The present invention meets these and other needs in the art.
It is an aspect of the present invention to transmit periodic constant bit rate data, such as telephone voice data, over a television cable network.
It is another aspect of the invention to maximize statistical multiplexing of data on the television cable network by minimizing the overhead needed for checking for off periods associated with periodic constant bit rate data.
Yet another aspect of the invention is to quickly respond to on periods of constant bit rate data in order to minimize delay.
Still another aspect of the invention is to utilize real time device polling to detect the onset of on periods of constant bit rate data over a television cable network.
A further aspect of the invention is to tightly synchronize the real time device polling with requests to send constant bit rate data over a cable network in order to minimize delay.
The above and other aspects of the invention are accomplished in a television cable network that employs a Cable Modem Termination System (CMTS) at the head end. The CMTS supports multiple user locations, some of which have a cable modem attached between the television cable and customer premises equipment (CPE), such as a telephone or personal computer.
The television cable network serves as a shared bus for both the downstream and upstream traffic, having a tree-and-branch architecture with analog transmission. The upstream channel is modeled as a stream of mini-slots. A mini-slot is the unit of granularity for upstream transmission opportunities.
The cable modems are slaves that transmit data in a Time Division Multiple Access (TMDA) sense based on allocation maps. The allocation map describes, for some interval, how the upstream mini-slots may be used. A given allocation map may describe some slots as grants for particular cable modems to use in transmitting data, other slots as available for contention transmission, and other slots as an opportunity for new cable modems to join the link.
Once a cable modem is recognized on the cable network, it is allocated a request slot. The request slot allows the cable modem to request a longer time slot for data transmission. The CMTS receives the request from the cable modem in the request slot, and allocates, or grants, a longer time slot to the requesting cable modem for data transmission in the next map sent downstream.
The codec (coder-decoder), in the cable modem or external to the cable modem, or other CBR source, has a certain framing periodicity. The codec converts a received analog signal into digital signals, and packs these signals into frames, suitable for network transmission. The real time device polling of the present invention polls the cable modems in an efficient way such that the latency associated with media access is minimized. Thus, the polling process runs in parallel with the codec generation of frames.